1. Field of the Invention
This invention relates to shear element type clutches for transmitting torque from a driving member to a driven member, and in particular to such a clutch which is responsive to an overload torque to disconnect the two members.
2. Discussion of the Prior Art
Clutches which transmit torque between a driving part and a driven part through ball type shear elements engaged with both parts are known, for example from U.S. Pat. Nos. 2,842,005, 4,429,775, 4,991,701 and 5,005,684. In these devices, torque is transmitted from one part to the other utilizing the shear strength of the ball type shear elements, which span the interface between the two parts when the clutch is engaged, and climb out of the interface when the clutch is disengaged. It is also known to provide a collar having a ramped surface engaging the balls and biasing the collar so as to bias the balls into the engaged position. See for example U.S. Pat. No. 4,429,779.
In some applications, such as in an agricultural drive line, the axial distance which may permissibly be occupied by an overload clutch is limited. In general, the shorter that the clutch is, the better.
In addition, it is sometimes desirable, such as in an agricultural driveline, to be able to vary the maximum torque which the clutch can transmit as the clutch is being operated, or to override the clutch, for example by releasing it from engagement. Since the prior art ball type overload clutches had springs, the only way that the overload torque could be varied was by changing the force exerted by the springs on the collar, for example by changing the springs. This was a time consuming and impractical process. In addition, with the prior overload clutches, it was not possible to remotely override the clutch. Moreover, in a torque overload condition, the prior art spring biased clutches would continually seek to reengage, resulting in violent chattering at some speeds and rough operation when the overload condition persisted. These attempts to re-engage also created heat which caused early failure of the clutch.